The invention is in the technical area of the production of devices for photovoltaic energy generation and relates to a method for the evaluative, quantitative analysis of photovoltaic layer systems.
Solar cells enable direct conversion of light radiation into electrical current. With respect to efficiency, thin-film solar cells based on polycrystalline chalcopyrite semiconductors have proved to be advantageous. In particular, copper indium diselenide (CuInSe2 or CIS) is distinguished by a particularly high absorption coefficient because of its band gap suited to the spectrum of sunlight. For adequate mechanical strength, thin-film solar cells require special carrier substrates, which, for the most part, contain inorganic glass, polymers, or metal alloys, and can, depending on the layer thickness and material properties, be implemented as rigid plates or flexible films. Since, typically, with individual solar cells, only voltage levels of less than 1 volt can be obtained, many solar cells are usually serially connected in a solar module in order to thus obtain a technically useful output voltage. For this, thin-film solar modules offer the particular advantage that the solar cells can already be connected serially in an integrated form during production of the films. To ensure lasting protection against environmental influences, the solar cells are customarily combined with low-iron soda lime glasses and adhesion-promoting polymer films into a weather-resistant composite.
However, during the production of solar modules, various kinds of defects can occur, which disadvantageously cause internal electrical power losses and thus reduce the efficiency of solar modules. Significant causes of such power losses are, for example, short circuits (shunts), which result in a locally elevated recombination rate of charge carriers, and relatively high series resistances, which result substantially from the ohmic resistances of metal contacts, feed lines, and semiconductor material as well as contact resistances of metal-to-semiconductor contacts. Moreover, mechanical defects, such as cracks, fractures, and delaminations or variations of material quality, can, for example, result in power losses.
In the series production of solar modules, it is important, in the context of satisfactory quality control, in particular to meet specific quality standards, to be able to identify solar modules with high internal power losses. It is known, for this purpose, to use special infrared measurement techniques in which an electric current is generated in the solar module and a thermal image of the surface of solar module is captured by an infrared camera. Since all basic processes in the solar cells are always associated with heat dissipation, and defects, such as short circuits and series resistances, are typically come along with relatively high power losses, these can be detected by a locally elevated temperature of the surface of the solar module. In the thermal image, the defects appear, for example, as brighter (warmer) spots (“hot spots”) or regions. In the scientific literature, this procedure has already been thoroughly described in many publications. Merely by way of example, reference is made to the technical article entitled “Quantitative Evaluation of Shunts in Solar Cells by Lock-in Thermography” by O. Breitenstein et al. in “Progress in Photovoltaics: Research and Applications” (Prog. Photovolt: Res. Appl. 2003; 11:515-526) and the citations mentioned therein. Additional technical background can be found in the patent applications U.S. 2010/201374 A1 and U.S. 2010/182421 A1.
In the series production of solar modules, the thermal images are usually assessed visually, with a qualitative statement made concerning their quality based substantially on the experience of the inspector.